What Does This Black Fused Junction Block Wire Do?

[Gregski]

Apologies for yet another: "what does this wire do question" LOL. I did pull down all the diagrams I could find and my truck is running and driving fine, however I am perplexed so I ask the question.

So what does the wire Arrow #1 in the diagram below do. Looks like it may be labeled "Engine Harness Connection" I ask because it does not connect to anything on the inside side of the firewall bulk head connector, Arrow #2.

I put this '71 GMC basket case truck together one piece at a time, so that's part of the reason I don't know this.

Full disclosure I converted to a one wire Alternator setup and again truck runs and drives fine without this black wire as I have deleted it, but I am just asking what was it for?

[VetteVet]

That's the battery gauge wire feed for the battery side of the gauge. There is another one for the alternator in the main harness.

Here is a lot more on the subject. Go to post 26 if you don't want to read the first part of the thread.
http://67-72chevytrucks.com/vboard/s...d.php?t=417872

[Gregski]

Originally Posted by VetteVet

That's the battery gauge wire feed for the battery side of the gauge. There is another one for the alternator in the main harness.

Here is a lot more on the subject. Go to post 26 if you don't want to read the first part of the thread.
http://67-72chevytrucks.com/vboard/s...d.php?t=417872

Thank you I followed the link and read that entire thread. OK, so I guess the graphic pictured above and the ones in the link are incomplete or are all missing a wire or something, because nothing plugs in to the bulk head in that spot and the graphic does not show cluster with gauges wiring just the wiring for a cluster with idiot lights. So when we follow the #8 Brown wire in the Cluster Connector with Idiot Lights we end up in the bulkhead connector where the brown wire shares a port with the brown and white wire. Not a big deal I just wish someone would make a diagram of one with the proper wire runs for the cluster with gauges.

[VetteVet]

Originally Posted by Gregski

Thank you I followed the link and read that entire thread. OK, so I guess the graphic pictured above and the ones in the link are incomplete or are all missing a wire or something, because nothing plugs in to the bulk head in that spot

and the graphic does not show cluster with gauges wiring just the wiring for a cluster with idiot lights.

So when we follow the #8 Brown wire in the Cluster Connector with Idiot Lights we end up in the bulkhead connector where the brown wire shares a port with the brown and white wire. Not a big deal I just wish someone would make a diagram of one with the proper wire runs for the cluster with gauges.

The reason this is so, is because the alternator needs a resistance wire to excite the fields to begin charging (stock factory). This is where the charging light comes into play. With the gauge cluster this brown wire is eliminated and only the resistor wire (brown/white) is present at the bulk head plug. It comes from the key switch ACC terminal shared with the 12 gauge brown wire that feeds the fuse panel accessories.


The reason the brown/white wire is there is in case the charging light bulb blows, the B/W wire will provide the resistance needed 10 ohms, to keep the alternator working.

As you have stated, the diagram shows the engine bay with the battery gauge wires and then on the inside of the cab it converts to the idiot light wiring.

This link shows the complete diagram except it shows just what I said. It sure is easy to read since it is expanded.
http://67-72chevytrucks.com/vboard/s...185856&page=13 post 302

If you notice the bulkhead view in the lower left corner of the second section(view A) it shows the two empty holes where the battery gauge wires would exit into the cab section if it were a gauge harness.

The key switch and the dash connectors are the main differences. Most of the wiring for the rest is the same.

The key switch has two brown wires and two green wires for the brake warning and the temperature warning lights. These wires are just grounds for the two lights to test them when starting the engine in case the bulbs are blown.

The cluster plug is different and is shown below.





The gauge diagram is shown with the correct wiring below.




The wiring pins correspond with the cluster plug I posted above.